doi: 10.1111/j.1751-3928.2011.00178.x Resource Geology Vol. 62, No. 1: 19–41 Thematic Articlerge_178 19..41 Mineralogy and Mineral Chemistry of the Cretaceous Duolong Gold-Rich Porphyry Copper Deposit in the Bangongco Arc, Northern Tibet Jinxiang Li, Guangming Li, Kezhang Qin,BoXiao, Lei Chen and Junxing Zhao Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China Abstract The Early Cretaceous Duolong gold-rich porphyry copper deposit is a newly discovered deposit with proven 5.38 Mt Cu resources of 0.72% Cu and 41 t gold of 0.23 g t-1 in northern Tibet. Granodiorite porphyry and quartz diorite porphyrite are the main ore-bearing porphyries.A wide range of hydrothermal alteration associated with these porphyries is divided into potassic, argillic and propylitic zones from the ore-bearing porphyry center outward and upward. In the hydrothermal alteration zones, secondary albite (91.5–99.7% Ab) occurs along the rim of plagioclase phenocryst and fissures. Secondary K-feldspar (75.1–96.9% Or) replaces plagioclase phenoc- ryst and matrix or occurs in veinlets. Biotite occurs mainly as matrix and veinlet in addition to phenocryst in the potassic zone. The biotite are Mg-rich and formed under a highly oxidized condition at temperatures ranging from 400°C to 430°C. All the biotites are absent in F, and have high Cl content (0.19–0.26%), with log (XCl/XOH) values of -2.74 to -2.88 and IV (Cl) values of -3.48 to -3.35, suggesting a significant role of chloride complexes - - (CuCl2 and AuCl2 ) in transporting and precipitating copper and gold. Chlorites are present in all alteration zones and correspond mainly to pycnochlorite. They have similar Fe/(Fe+Mg), Mn/(Mn+Mg) ratios, and a formation temperature range of 280–360°C. However, the formation temperature of chlorite in the quartz- gypsum-carbonate-chlorite vein is between 190°C and 220°C, indicating that it may have resulted from a later stage of hydrothermal activity.Fe3+/Fe2+ ratios of chlorites have negative correlation withAlIV, suggesting oxygen fugacity of fluids increases with decreasing temperature. Apatite mineral inclusions in the biotite phenocrysts show high SO3 content (0.44–0.82%) and high Cl content (1–1.37%), indicating the host magma had a high oxidation state and was enriched in S and Cl. The highest Cl content of apatite in the propylitic zone may have resulted from pressure decrease, and the lowest Cl content of apatite in the argillic zone may have been caused by a low Cl content in the fluids. The low concentration of SO3 content in the hydrothermal apatite compared to the magmatic one may have resulted from the decrease of oxygen fugacity and S content in the hydrothermal fluid, which are caused by the abundant precipitation of magnetite. Keywords: Bangongco arc, Duolong, gold-rich porphyry copper deposit, mineral chemistry, mineralogy. 1. Introduction Bureau of Tibetan Geology and Exploration in 2000, together with a super-large prospect (proven Cu The Duolong gold-rich porphyry copper deposit was resources of 5.38 Mt, 0.72% Cu; 41 t gold, 0.23 g t-1 Au) recently discovered by the No.5 Geological Team of the in the Bangongco arc in northern Tibet. At this deposit, Received 28 June 2010. Accepted for publication 26 October 2011. Corresponding authors: G. LI and J. LI, Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. Emails: [email protected], [email protected] © 2011 The Authors Resource Geology © 2011 The Society of Resource Geology 19 J. Li et al. 7878° 8282° 8686° 9090° 9494° 9898° AKSAKS JSSJSS A IYSIYS Songpan-Ganzeo Indus-Yarlung suture ng pa ° n-G 3434 BNSBNS Bangong-Nujiang suture FigFig 1B1B an ze QiangtangQiangtang terraneterrane TeTe rranerra JSSJSS Jinshajiang suture GangarGangar ne BNB ChangduChangdu GerzeGerze NS AKSAKS Ayimaquin-Kunlun s utute LhasaLhasa terraneterrane Duolong copper deposit CoqenCoqen 3030° IYSIYS LhasaLhasa Gangdese copper belt ZhongbaZhongba SutureSuture XigazeXigaze Yulong copper belt Stike-slipeStike-slipe faultfault ThustThust beltbelt DetachmentDetachment faultfault IndiaIndia 0 200km200km 2626° B DbzJ2-2DbzJ2-2 Zk001Zk001 N W E Zk3104Zk3104 Zk0802Zk0802 S Zk1502Zk1502 Zk002Zk002 QuaternaryQuaternary NeoceneNeocene KangtuoKangtuo groupgroup LateLate CretaceousCretaceous MeiriqieMeiriqie groupgroup MiddleMiddleJ Jurassicurassic YYanshipinganshiping g grouproup Post-mineralizationPost-mineralization basalticbasaltic andesiteandesite SecondarySecondary enrichmentenrichmentz zoneone PrePre oror syn-mineralizationsyn-mineralization basalticbasaltic andesiteandesite OreOreb bodyody UndistinguishedUndistinguished ore-formingore-forming granodioritegranodiorite FaultFault pporphyryorphyry a andnd q quartzuartz d dioriteiorite p porphyriteorphyrite FreshFresh granodioritegranodiorite porphyryporphyry DrillDrill holehole 0 1000m ModerateModerate argillicargillic zonezone SampleSample positionposition Fig. 1 Sketch tectonic and location map (a) (after Hou et al., 2004) and the generalized geologic map (b) of the Duolong gold-rich porphyry copper deposit (Li et al., 2008). abundant magnetite is associated with Cu-Au mineral- sition of the secondary K-feldspar, albite, chlorite, ization. This deposit is related to relatively mafic quartz apatite, biotite, magnetite, rutile and sulfide in the diorite porphyrite and granodiorite porphyry in a Duolong deposit by systematic electron-probe micro- magmatic arc setting (Li et al., 2007, 2012; Li et al., 2008). scopic analyses, and discuss their geochemical and pet- Due to the discovery of the Duolong deposit, the rological significance and the evolution of magmatic- Bangongco Mesozoic arc now becomes the third por- hydrothermal fluid. phyry copper belt with potential epithermal Au min- eralization (Qin et al., 2006) (Fig. 1a) following the 2. Geological settings and ore Yulong (Hou et al., 2003, 2007) and Gangdese porphyry deposit geology copper belts (Hou et al., 2004, 2009; Qin et al., 2005, 2008) in the Qinghai-Tibet plateau. Although the The Duolong gold-rich porphyry copper deposit is Duolong deposit has been studied in terms of fluid located roughly 100 km northwest of Gereze county inclusions characteristics (Li et al., 2007), geochronol- at the north of the Bangongco–Nujiang suture zone ogy and geochemistry (Li et al., 2008), no investigation (Fig. 1a). This deposit formed in a Cretaceous con- focusing on the hydrothermal minerals has been con- tinental arc setting (Qin et al., 2006; Li et al., 2008). The ducted. In this study, we describe the chemical compo- ore-bearing granodiorite porphyry yields a zircon © 2011 The Authors 20 Resource Geology © 2011 The Society of Resource Geology Mineralogy and mineral chemistry SHRIMP U-Pb age of 121.6 Ϯ 1.9 Ma (Li et al., 2008), outward and upward (Fig. 2). However, the phyllic and secondary K-feldspar yields a 40Ar-39Ar plateau alteration is not well developed and sericite–quartz age of 115.2 Ϯ 1.1 Ma (Li et al., 2011a). These ages indi- veins occur only in the local area. cate that this deposit formed in the Early Cretaceous Potassic alteration consisting of K-feldspathization and resulted from the subduction of the Neo-Tethys and biotitization is dominated in the deep part of the Ocean. The strata consist mainly of the Middle Jurassic granodiorite porphyry (Fig. 2) and is superimposed Yanshiping Group, the Late Cretaceous Meiriqie by argillic alteration. Secondary K-feldspar replaced Group, the Early Tertiary Kangtuo Group and the mainly plagioclase and dispersive matrix. Albite Quaternary (Fig. 1b). The Middle Jurassic Yanshiping reaction rim on the plagioclase often occurred as a Group is a clastic-interbedded volcanic sequence result of Na precipitation during the process of of littoral facies, which strikes east-west and dips K-feldspar metasomatism. K-feldspar alteration 50–80°WNW. It is composed of feldspar quartz sand- halos also occur at the edge of quartz-chalcopyrite- stone, siltstone-interbedded siliceous rock, basalt and magnetite veins (A-type, Gustafson & Hunt, 1975). dacite. The Late Cretaceous Meiriqie Group contains Additionally, the quartz–K-feldspar veinlets occur basaltic andesite, dacite, volcanic-clastic rocks, in the intrusive rocks. Secondary biotite replaces phenocryst-rich andesite and andesite. The Early Ter- mainly primary hornblende and biotite. Quartz- tiary Kangtuo Group is composed of the brown-red biotite-chalcopyrite veins and biotite veinlets (EB clay and sandy gravel. type, Gustafson & Quiroga, 1995) are also present. Hydrothermal magnetite occurs in the potassic alter- 2.1 Magmatic activity ation zone, where chalcopyrite exists with magnetite. Petrographic observations suggest that magnetite The Early Cretaceous hypabyssal volcanic rocks formed at the same time as the chalcopyrite or earlier mainly contain granodiorite porphyry and quartz than it. diorite porphyrite, which intruded into the Middle The argillic alteration zone contains mainly kaolinite, Jurassic Yanshiping Group. These intrusive rocks illite and muscovite, replacing plagioclase and chlorite occur as stock and dike, and host mineralization replacing biotite. The quartz-chalcopyrite (present in (Fig. 1b). They are exposed to the surface in the the center of the vein) veins (B type, Gustafson & Hunt, northeastern and southeastern portions of the ore dis- 1975) and chalcopyrite veinlets occur in the argillic trict. The granodiorite porphyry shows a spindly alteration. shape (width of 200 m and length